This invention relates generally to photovoltaic devices. More specifically, the invention relates to photovoltaic modules comprised of two or more electrically interconnected photovoltaic cells. Most specifically, the invention relates to photovoltaic power generating modules, and methods for their manufacture, wherein the number of electrical interconnections between the cells comprising the module are minimized.
Photovoltaic devices are nonpolluting and silent in operation. They are readily adapted to either a centralized or distributed power generating system, and growing in popularity as an alternative to fossil or nuclear powered electrical generation systems. The high cost and small size of single crystal photovoltaic devices has heretofore limited their utility; however, high volume processes for the manufacture of thin film semiconductor devices are now in commercial production, and very large area photovoltaic materials are being manufactured in continuous, roll-to-roll processes. The photovoltaic material produced in such processes is generally cut to size and fabricated into power generating modules which are comprised of a number of electrically interconnected photovoltaic cells selected to provide a desired output voltage and current. Such modules should have high power generating efficiency, and toward that end it is desirable to maximize the photovoltaically active surface of each cell. In addition, such modules should be rugged, low in cost and easy to fabricate. In addition, many applications require that such power generating modules be as light in weight as possible.
In view of these criteria, the prior art has developed a number of processes for fabricating large area photovoltaic materials into individual power generating cells and/or modules comprised of interconnected cells. U.S. Pat. Nos. 5,457,057; 5,681,402; and 5,759,291, the disclosure of which are incorporated herein by reference, show particular configurations of photovoltaic cells in which current collecting grid wires are employed to convey photovoltaically generated currents to a bus bar member. The disclosed structures maximize the photovoltaically active area of the devices; and in addition, are reliable, rugged and easily fabricated.
Referring now to FIGS. 1 and 2, there is shown a photovoltaic cell 20 of the prior art, which is generally similar to cells specifically disclosed in the U.S. Pat. No. 5,759,291 patent. FIG. 1 is a top plan view of the cell 20, and FIG. 2 is a cross-sectional view of that cell 20 taken along line 2xe2x80x942 of FIG. 1. As is best seen in FIG. 2, the cell 20 includes a substrate electrode 22 having a body of photovoltaic material 24 disposed upon a topmost surface thereof. A top electrode 26 is disposed upon the upper surface of the photovoltaic body 24.
The photovoltaic body 24 is typically comprised of a number of layers of semiconductor material, and it operates to absorb incident photons and generate electron-hole pairs which are collected by the substrate electrode 22 and top electrode 26 to generate a photocurrent. The patents incorporated by reference herein describe some specific materials and configurations for the photovoltaic body; although, it is to be understood that the present invention may be implemented using various photovoltaic materials.
The substrate electrode 22 is, in some instances, fabricated from a sheet of electrically conductive material, such as a sheet of stainless steel, aluminum, electrically conductive polymer, cermet, degenerate semiconductor or the like, and it may include further layers thereupon, such as light reflective layers, texturizing layers and the like as is known in the art. In other instances, the substrate 22 may comprise a composite substrate such as a body of polymeric material having an electrically conductive coating, such as a coating of metal, non-metallic conductor or the like thereupon. Such composite substrates are particularly advantageous where the weight of the finished device is a concern. Again, the present invention may be implemented with all of such substrates. The top electrode 26 is transmissive of photovoltaically active wavelengths. One particularly preferred group of top electrode materials comprises transparent, electrically conductive oxide (TCO) material such as indium oxide, tin oxide, mixed oxides of indium and tin, and the like.
As is also seen in FIG. 2, a current collecting grid wire 28 is disposed upon the top, light incident, surface of the top electrode 26. This grid wire serves to collect photogenerated currents and convey them to a pair of current collecting bus bars 30a, 30b. Inclusion of this grid wire 28 is important, since the electrical conductivity of most TCO materials is fairly low; hence, the series resistance of the photovoltaic device 20 would be very high if the photocurrents had to travel laterally very far through the top electrode material. The grid wire 28 shortens this current path and provides a high conductivity conduit for the photo currents.
As further illustrated in FIG. 2, the bus bars 30a, 30b, as well as the end portions of the grid wire 28, are supported by an electrically insulating body 32a, 32b disposed upon the semiconductor body 24. This insulating body prevents the bus bar 30 and grid wire 28 from short circuiting through to the substrate 22. In one particularly preferred embodiment, and as is described in the ""291 patent, this insulating body 32 is comprised of a double-sided adhesive tape which is applied to the semiconductor body 24 prior to the deposition of the top electrode material 26 thereupon. In other embodiments, and as is described in the patents incorporated by reference herein, this insulating body 32 may be disposed upon the substrate 22. Also, in some instances, the bus bars 30 may be directly affixed to the semiconductor body 24, and the grid wires bonded between the bus bar and semiconductor or atop the semiconductor body.
Referring now to FIG. 1, there is shown a top plan view of the device of FIG. 2. Visible in the figure is the top electrode material 26, grid wires 28a-28e, bus bars 30a, 30b and edge portions of the substrate 22.
As is described in the patents incorporated by reference herein, the photovoltaic cell material of FIGS. 1 and 2 can be electrically interconnected to form multi-cell modules. Such modules are typically configured as a series connected group of cells, and in that regard, an electrical connector is employed to establish a current path between the bus bar of one cell and the substrate of an adjacent cell. These interconnectors are typically soldered or welded, and FIGS. 11 -13 of the ""057 patent illustrate such interconnections. As is known and described in the art, the interconnected cells are typically laminated with an encapsulant material to produce a finished module.
While prior art technology as described hereinabove has produced high quality, high efficiency modules, the inventors hereof have recognized that the steps and structures associated with the formation of interconnections in accord with the prior art can be improved upon so as to maximize the efficiency, reliability and ease of fabrication of such photovoltaic modules.
As will be described in detail hereinbelow, the present invention provides a photovoltaic module in which the number of soldered or welded joints in the module is minimized thereby decreasing the likelihood of device failure and simplifying its manufacture. In addition, the structures of the present invention provide a highly redundant current path through the device which further enhances its reliability. The structures of the present invention are readily adaptable to the manufacture of very lightweight photovoltaic generator devices having ultra lightweight composite substrates. These and other advantages of the present invention will be apparent from the drawings, discussion and description which follow.
There is disclosed herein a photovoltaic module which is comprised of a first and a second photovoltaic cell. Each cell has a substrate electrode, a top electrode, and a photovoltaic semiconductor body disposed therebetween in electrical communication with the substrate electrode and the top electrode. Each cell further includes a plurality of current collecting grid wires disposed atop, and in electrical contact with, the top electrode. The grid wires of a first cell are in electrical communication with a current collecting bus bar, and the grid wires of a second cell extend onto, and establish electrical contact with, the substrate electrode of the first cell so as to establish an unbroken current path therebetween and thereby create a series electrical connection between the first and the second cell. In a like manner, further cells may be added to the module.
In specific embodiments, the grid wires are adhesively affixed to the top electrode by an electrically conductive adhesive, and this adhesive may be precoated onto the exterior surface of the grid wires. In particular embodiments, the grid wire comprises a metallic core, which may be of circular or noncircular cross section, and one preferred core material comprises silver plated copper. The module may further include an encapsulant which protects and supports the interconnected cells.
In some configurations, the grid wires of the second cell extend onto an upper surface of the grid wires of the first cell; while in other instances, the grid wires of the second cell may contact the bottom surface of the substrate of the first cell. The present invention may be implemented in connection with prior art grid wire methodology; and in that regard, the grid wires may pass onto an electrically insulating body disposed on each cell, in which instance, the bus bar of the first cell may be affixed to that insulating body in electrical contact with the grid wires thereof.
Also disclosed herein are methods for manufacturing the photovoltaic modules of the present invention, and such methods comprise providing a first and a second photovoltaic cell each comprised of a substrate electrode, a top electrode and a photovoltaic semiconductor body disposed therebetween in electrical communication with the substrate electrode and the top electrode. According to this method, a plurality of current collecting grid wires are disposed atop the top electrode of the first cell in electrical contact therewith, and a current collecting bus bar is disposed so as to establish electrical contact with the grid wires of the first cell. A plurality of grid wires are affixed to the top electrode of the second cell so as to establish electrical contact therewith and the wires of this plurality are extended from the second cell to the substrate electrode of the first cell so as to establish electrical contact therewith. The wires of the second plurality each provide an unbroken current path between the two cells so as to establish a series electrical connection therebetween.